Papers by Author: W.C. Mitchel

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Authors: W.C. Mitchel, J. Brown, D. Buckanan, R. Bertke, K. Malalingham, F. Orazio, P. Pirouz, Huang-Ju R. Tseng, Uma B. Ramabadran, Bahram Roughani
Authors: W.C. Mitchel, William D. Mitchell, S.R. Smith, G.R. Landis, A.O. Evwaraye, Z.Q. Fang, David C. Look, J.R. Sizelove
Abstract: A variety of 4H-SiC samples from undoped crystals grown by the physical vapor transport technique have been studied by temperature dependent Hall effect, optical and thermal admittance spectroscopy and thermally stimulated current. In most samples studied the activation energies were in the range 0.9 - 1.6 eV expected for commercial grade HPSI 4H-SiC. However, in several samples from developmental crystals a previously unreported deep level at EC-0.55 ± 0.01 eV was observed. Thermal admittance spectroscopy detected one level with an energy of about 0.53 eV while optical admittance spectroscopy measurements resolved two levels at 0.56 and 0.64 eV. Thermally stimulated current measurements made to study compensated levels in the material detected several peaks at energies in the range 0.2 to 0.6 eV.
Authors: Mary Ellen Zvanut, Won Woo Lee, Hai Yan Wang, W.C. Mitchel, William D. Mitchell
Abstract: The high resistivity of SiC required for many device applications is achieved by compensating residual donors or acceptors with vanadium or intrinsic defects. This work addresses the defect levels of substitutional vanadium and the positively charged carbon vacancy (VC +) in semiinsulating (SI) SiC. After reviewing the earlier studies related to both defects, the paper focuses on temperature-dependent Hall measurements and photo-induced electron paramagnetic resonance (EPR) experiments of 4H and 6H SI SiC. In vanadium-doped samples, a V3+/4+ level near Ec-1.1 eV (4H) and Ec-0.85 eV (6H) is estimated by a comparison of dark EPR spectra and the activation energy determined from the Hall data, assuming that vanadium controls the Fermi level. In high purity semiinsulating substrates, analysis of time-dependent and steady-state photo-EPR data suggests that the plus-to-neutral transition of the carbon vacancy involves a structural relaxation of about 0.6 eV.
Authors: W.C. Mitchel, Ronald Perrin, Jonathan Goldstein, Matthew D. Roth, M. Ahoujja, S.R. Smith, A.O. Evwaraye, J.S. Solomon, G. Landis, Jason R. Jenny, H. McD. Hobgood, G. Augustine, Vijay Balakrishna
Authors: Z.Q. Fang, B. Claflin, David C. Look, L. Polenta, J. Chen, Thomas Anderson, W.C. Mitchel
Abstract: Thermally stimulated current spectroscopy (TSC) has been applied to characterize deep traps in high-purity semi-insulating 6H-SiC substrates. By using above bandgap to sub-bandgap light for illumination at 83 K and different applied biases, at least nine TSC traps in the temperature range of 80 to 400 K can be consistently observed. It is found that TSC peaks for T < 130 K are significantly affected by light and some peaks are strongly enhanced by the applied bias. Measured trap activation energies range from 0.15 eV to 0.76 eV. Theoretical fittings of selected traps give more accurate trap parameters. Based on literature results connected with deep traps in conductive 6H-SiC, the origin of these TSC traps is discussed.
Authors: Adam W. Saxler, K.S. Kim, D. Walker, P. Kung, Xue Ren Zhang, G.J. Brown, W.C. Mitchel, M. Razeghi
Authors: Ekaterina N. Kalabukhova, S.N. Lukin, D.V. Savchenko, W.C. Mitchel, Siegmund Greulich-Weber, Uwe Gerstmann, Andreas Pöppl, J. Hoentsch, E. Rauls, Yurii Rozentzveig, E.N. Mokhov, Mikael Syväjärvi, Rositza Yakimova
Abstract: D-band electron paramagnetic resonance (EPR) measurements as well as X and Q-band field-swept Electron Spin Echo (ESE) and pulsed Electron Nuclear Double Resonance (ENDOR) studies were performed on a series of n-type 4H-SiC wafers grown by different techniques including sublimation sandwich method (SSM), physical vapor transport (PVT) and modified Lely method. Depending on the C/Si ratio and the growth temperature the n-type 4H-SiC wafers revealed, besides a triplet due to nitrogen residing on the cubic site (Nc), two nitrogen (N) related EPR spectra with g||=2.0055, g⊥=2.0010 and g||=2.0063, g⊥=2.0005 with different intensities. In the samples with low C/Si ratio the EPR spectrum with g|| =2.0055, g⊥=2.0010 consists of a triplet with low intensity which is tentatively explained as a N-related complex, while in the samples with high C/Si ratio the triplet is transformed into one structureless line of high intensity, which is explained as being due to an exchange interaction between N donors. In the samples grown at low temperature with enhanced carbon concentration the EPR line with g||=2.0063, g⊥=2.0005 and a small hyperfine (hf) interaction dominates the EPR spectrum. It is attributed to N on the hexagonal lattice site. The interpretation of the EPR data is supported by activation energies and donor concentrations obtained from Hall effect measurements for three donor levels in this series of 4H-SiC samples.
Authors: Howard E. Smith, Kurt G. Eyink, W.C. Mitchel, M.C. Wood, Mark A. Fanton
Abstract: A multiple data point version of the industry standard, two data point raster-changing procedure is employed to measure low levels (< 1 x 1017 atoms/cm3) of nitrogen (N) in silicon carbide (SiC) by SIMS (Secondary Ion Mass Spectrometry). A current-changing procedure is also employed. Together, these are used evaluate the assumptions of the standard method, to separate and measure the components of background signal, and to improve upon the precision and accuracy of the standard method. The risk of poor precision in the two-point method is demonstrated, as is the improvement provided by the multiple-point method. Results show that, in addition to the wellknown N memory background, adsorption background can contribute significantly to the N signal. In general, establishing the presence of adsorption gas in this way can be used to warn of the presence of ionization background, which is not measurable per se.
Authors: Adam W. Saxler, P. Kung, Xue Ren Zhang, D. Walker, J. Solomon, M. Ahoujja, W.C. Mitchel, H.R. Vydyanath, M. Razeghi
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